The present invention relates to a method of programming a non-volatile memory device. More particularly, the present invention relates to a program-verify method.
In general, there is an increasing demand for non-volatile memory devices which can be electrically programmed and erased and do not need the refresh function of rewriting data at regular intervals. The term “program” refers to the operation of writing data into the memory cells.
To increase the level of integration of memory devices, a NAND flash memory device in which a plurality of memory cells are connected in series (i.e., a structure in which neighboring cells share the drain or source) to form one string has been developed. The NAND flash memory device is a memory that sequentially reads information unlike the NOR flash memory device.
The distribution of threshold voltages (Vt) for cells being programmed (or program cells) in conventional NAND flash memory device at least partly determines the device performance, e.g., read margin. The threshold voltage (Vt) of the program cell is controlled by an incremental step pulse program (ISPP) for certain NAND flash memory device. More particularly, in the NAND flash memory device having multi-level cells, the distribution of the threshold voltage (Vt) of the program cell is one of the important factors that determines the device performance.
FIG. 1 shows a memory cell array of a general NAND flash memory device. In such a NAND flash memory device, if the ISPP is used, the cells MC0 to MC31 in a string may have slightly different threshold voltages (Vt) due to a back pattern dependency (BPD) effect and an interference effect. This causes the distribution of the threshold voltages (i.e., the differences in threshold voltages) of the memory cells to widen. This increased threshold voltage differences among memory cells may degrade a read margin in single level cells or multi-level cells and may also have an adverse effect on margin in-cycling and retention characteristics.
FIG. 2 is a graph showing the distribution of threshold voltages of the program cells using the ISSP.
Referring to FIG. 2, the program cell MC31 of the word line WL31 that is programmed last is not influenced by an interference cell threshold voltage (a state where threshold voltages of access cells are distorted according to threshold voltages of neighboring cells). Accordingly, it can be seen that the program cell MC31 of the word line WL31 has the distribution of a threshold voltage in a region different from those of the remaining program cells MC0 to MC30. Therefore, the threshold voltages of the program cells MC0 to MC31 in a string vary significantly; i.e., the cells have the threshold voltage distribution about 1.4 V.